In recent years, a liquid crystal alignment film produced by an orientation treatment of a liquid crystal polymer or a liquid crystalline compound having a polymerizable functional group has been developed as an optical film for an optical compensator and the like used for a liquid crystal display. This film attracts attention as being capable of providing a high degree of orientation, i.e. oblique orientation, torsion orientation, and the like, which cannot be obtained by a birefringence film utilizing the polymer film stretching technology.
A cholesteric polarizer utilizing selective reflection characteristics of a liquid crystal alignment film (selective reflection film) obtained by cholesteric orientation of a compound comprising a liquid crystal polymer or a polymerizable liquid crystal compound such as a liquid crystalline (meth)acrylate compound and a chiral compound has also been put on practical use.
The selective reflection center wavelength (λ) which is one of the selective reflection characteristics is shown by the formula λ=n×P (wherein n is the average refractive index and P is a cholesteric pitch). The selective reflection wavelength zone (Δλ) is shown by the formula Δλ=Δn×P (wherein Δn is (ne−no), wherein ne indicates an extraordinary light refractive index and no indicates an ordinary light refractive index). Therefore, in order to expand the selective reflection wavelength zone (Δλ), a material having a large Δn is desired.
In order to use a selective reflection film as a cholesteric polarizer for a liquid crystal display, the film must exhibit selective reflection in the visible ray region. Since the selective reflection wavelength zone Δλ in one layer of a selective reflection film is usually narrower than a visible ray region, two or more selective reflection films are laminated in order to broaden the selective reflection wavelength zone Δλ. For this reason, the selective reflection film using a material having a narrow selective reflection wavelength zone Δλ requires a large number of layers of lamination, resulting in low productivity. Therefore, a material (such as a polymerizable liquid crystal compound) having a large Δn, that is, a material having a wide selective reflection wavelength zone Δλ, has been desired.
However, generally known polymerizable compounds having a large Δn exhibits poor solubility, applicability, and orientation properties. Some of them cannot produce a uniform film and have difficulty in providing a usable selective reflection film.
Azines shown by the following formula (A) have been known for many years and used as liquid crystalline compounds.
wherein Ra represents an alkyl group and Rb represents an alkyl group, a cyano group, a fluorine atom, a trifluoromethoxy group, or the like.
These compounds are liquid crystalline materials having excellent properties such as a high liquid crystal phase upper limit temperature and being comparatively chemically stable and inexpensively produced.
However, these azines are not necessarily satisfactory as to mutual solubility with general purpose liquid crystal compounds used at the present time. Although the mutual solubility can be improved to a certain extent by increasing the number of carbon atoms in the side chain alkyl group in the formula (A), such a compound has a low liquid crystal phase upper limit temperature.
In order to solve such a problem, a liquid crystalline compound shown by the following formula (B) has been proposed in Patent Document 1.
wherein R is a hydrogen atom or an alkyl group having 1 to 12 carbon atoms, the double bond is in a trans displacement when R is an alkyl group, m represents an integer of 1 to 10, n is 0 or 1, W, X, and Y individually represent a fluorine atom, a chlorine atom, a methyl group, a cyano group, or a hydrogen atom, and Z represents a fluorine atom, a chlorine atom, a cyano group, an alkyl or alkoxyl group having 1 to 12 carbon atoms, or an alkenyl or alkenyloxy group having 3 to 12 carbon atoms, wherein one or more hydrogen atoms in these groups may be substituted with fluorine atoms.
These compounds are chemically stable to heat, light, and the like, have excellent liquid crystallinity, and can be easily manufactured industrially. Since the compounds have excellent mutual solubility with general liquid crystalline compounds and crystalline compositions, the crystal response time can be significantly improved by using the compounds. Therefore, the compounds are usable as components of a liquid crystalline material for a liquid crystal display element having a wide temperature range and being capable of responding promptly.
However, performance of liquid crystal display devices is advancing day by day in recent years, demanding development of a liquid crystalline material having a higher liquid crystal phase upper limit temperature, which is chemically stable, can be inexpensively produced, and exhibits a large Δn.
Patent Document 1: JP-A-10-147562 (U.S. Pat. No. 6,010,642)